1. Field of the Invention
In the drilling and production of wells for the recovery of petroleum and other subsurface deposits, it is frequently necessary to close off or plug a tubular conduit, such as a string of tubing extending into the well, at a chosen point along the length of the conduit so that pressure may be built up within the conduit above that point. Subsequently, it is necessary to be able to re-open the conduit for flow therethrough. For example, it is periodically necessary to pack off the annulus between the tubing and a surrounding casing or the like. This is typically done with a packer which is made up into and forms a part of the tubing string. Many such packers are set hydraulically by application of fluid pressure through the tubing. Therefore, a plug apparatus of the type generally described above may be used to permit sufficient pressure to be built up within the tubing to set the packer. Such packers are typically designed so that, once set, they may be held or retained in the set condition without the continued application of fluid pressure through the tubing. Thus, the plug used to close off the tubing during setting of the packer may then be released so that fluid may be circulated through the tubing.
2. Description of the Prior Art
One of the simplest types of plug apparatus for such purposes includes a generally annular seat member which is mounted coaxially within the tubing string by shear pins or the like. When it is desired to close the tubing, a plug is pumped downwardly through the tubing until it is caught on the seat, which is suitably sized and configured to receive it. This closes off the opening through the annular seat member, and thus the tubing conduit, and allows pressure to be built up above the plug. When it is desired to re-open the tubing, the pressure is further increased, shearing the pins mounting the seat member in the tubing and permitting the seat member and plug to be pumped out through the lower end of the tubing. In other devices, the seat may be a permanent part of the tubing string, but the plug may be compressible so that, by increasing the pressure upstream of the plug, it may be pumped through the seat member thereby reopening the latter.
In either of these two types of plug apparatus, the release of the plug and/or seat allows a sudden or extremely rapid decrease of the pressure which had theretofor built up above the plug. This in turn produces a shock wave along the tubing string which may result in various ill effects, including kinking of the tubing, unsetting of a previously set packer, and other types of damage or malfunction in the well apparatus. A similar shock, due to the rapid increase in pressure, occurs below the plug and may damage the formation. Accordingly, it has been desired to develop a plug apparatus which could be released without producing a sudden shock wave.
U.S. Pat. No. 3,090,442, U.S. Pat. No. 3,331,378, and U.S. Pat. No. 4,114,694 all disclose plug assemblies which accomplish this purpose by designs wherein release of the plug apparatus is accomplished by a decrease, rather than an increase, in the pressure being applied to the tubing above the plug. By releasing the pressure gradually in a controlled manner, the aforementioned shock wave can be avoided. However, depending on various factors such as the well conditions, the apparatus being used in the well, etc., the pressure may be inadvertently decreased, as by failure or leakage of some part of the tubing string, pump failure, or the like. Consequently, the plug may be released prematurely, and the cost, both in terms of time and money, of such an accident can be considerable.
Other systems have involved a gradual decrease in pressure at the top of the well in an effort to avoid shock waves. However, this technique does not always result in a sufficient reduction in the downhole pressure differential, i.e. at the plug, to effect the desired result.